Tracking filter for sensing DC content in an AC waveform

Abstract

The DC content of an AC waveform is detected by a digital filter which partially filters each phase of the waveform and applies the filtered waveforms sequentially through a multiplexer to a sample and hold circuit which measures the instantaneous value of each phase at two points which are spaced an exact odd number of half cycles apart. The time interval between the measurement points is adjusted according to variations in the AC waveform frequency to maintain required spacing. The measured instantaneous values are fed by an analog to digital converter and to a microprocessor system which averages the two readings to generate a signal corresponding to the DC content for each phase.

What is claimed is: 
     
       1. A method of determining the DC content in an AC waveform which is subject to frequency variations, said method comprising the steps of: measuring a first instantaneous value of said AC waveform at a first point in a cycle of the AC waveform;   storing a signal representative of said first instantaneous value;   measuring the frequency of said AC waveform;   determining a time interval which is substantially equivalent to an exact odd number of half cycles of said AC waveform at the measured frequency;   measuring the instantaneous value of said AC waveform at a second point in the waveform separated by said time interval from said first point, wherein said instantaneous values of said AC waveform at said first and second points are equally and oppositely displaced from the mid-value of said AC waveform; and   generating a signal equal to the DC content of said AC waveform by averaging said first and second instantaneous values of the AC waveform.   
     
     
       2. The method of claim 1, wherein said step of determining a time interval comprises the steps of: subtracting the measured frequency of said AC waveform from a preselected frequency to obtain a frequency deviation;   multiplying said frequency deviation by a preselected scaling factor to obtain an incremental count;   adding said incremental count to a preselected count in a programmable counter to obtain a starting count; and causing said programmable counter to count during one half cycle of said AC waveform to achieve a final count which is proportional to a substantially exact odd number of half cycles of said AC waveform at the measured frequency.   
     
     
       3. The method of claim 1, wherein said first point is a random point in said AC waveform. 
     
     
       4. The method of claim 1, further comprising the steps of: filtering said AC waveform prior to measuring said instantaneous values.   
     
     
       5. The method of claim 4, further comprising the step of: adding a bias voltage to the filtered AC waveform prior to measuring said instantaneous values, said bias voltage being of sufficient magnitude to ensure that the first and second instantaneous values will be of the same polarity; and   wherein the averaging step includes adding the two biased instantaneous values together and subtracting a signal equal to twice the bias voltage from the sum.   
     
     
       6. A method of determining the DC content in a plural phase AC waveform which is subject to frequency variations, said method comprising the steps of: successively measuring a first instantaneous value of each phase of the AC waveform at a first point in each phase;   storing signals representative of said first instantaneous value of each phase of the AC waveform;   measuring the frequency of the AC waveform;   determining a time interval which is substantially equivalent to an exact odd number of half cycles of the AC waveform at the measured frequency;   successively measuring the instantaneous value of each phase of the AC waveform at respective second points in the AC waveform separated by said time interval from the respective first points, wherein said instantaneous values of each phase of the AC waveform at the first and second points are equally and oppositely displaced from the mid-value of the associated phase of the AC waveform; and   generating a signal equal to the DC content of each phase by averaging the instantaneous values of the associated phase at the associated first and second points.   
     
     
       7. The method of claim 6, wherein said step of determining a time interval comprises the steps of: subtracting the measured frequency of the AC waveform from a preselected frequency to obtain a frequency deviation;   multiplying said frequency deviation by a preselected scaling factor to obtain an incremental count;   adding said incremental count to a preselected count in a programmable counter to obtain a starting count; and causing said programmable counter to count during one half cycle of said AC waveform to achieve a final count which is proportional to a substantially exact odd number of half cycles of said AC waveform at the measured frequency.   
     
     
       8. The method of claim 6, wherein said first points are random points in each phase of said AC waveform. 
     
     
       9. The method of claim 6, further comprising the step of: filtering said AC waveform prior to measuring said instantaneous values.   
     
     
       10. The method of claim 9, further comprising the step of: adding a bias voltage to the filtered AC waveform prior to measuring said instantaneous values, said bias voltage being of sufficient magnitude to ensure that all instantaneous values are of the same polarity; and   wherein the averaging step includes adding the two biased instantaneous values for each phase together and subtracting a signal equal to twice the bias voltage from the sum.   
     
     
       11. An apparatus for generating a signal representative of the DC content of an AC waveform which is subject to frequency variations, said apparatus comprising: means for measuring the instantaneous value of the AC waveform;   means for storing said instantaneous value;   means for determining a time interval which is substantially equal to an exact odd number of half cycles of the AC waveform; and   control means for operating the measuring means and storing means to measure the instantaneous value of the AC waveform at said first point in time and to store the measured value, and for subsequently operating the measuring means to measure the instantaneous value of the AC waveform at a second point in time spaced from the first point by said time interval, wherein said instantaneous values of the AC waveform at said first and second points are equally and oppositely displaced from the mid-value of the AC waveform, and for generating a DC content signal by averaging the first and second measured instantaneous values.   
     
     
       12. An apparatus as recited in claim 11, wherein said AC waveform is multiphased and wherein said control means includes: means for operating the measuring means to successively measure the instantaneous value of each phase of the AC waveform at a first point in a cycle of each phase and to store the instantaneous value in said storage means, for subsequently operating said measuring means to measure the instantaneous value of each phase of the AC waveform at a second point in time spaced from said first point by said time interval, wherein said instantaneous values of each phase of said AC waveform at said first and second points are equally and oppositely displaced from the midvalue of the associated phase of said AC waveform, and to store the measured instantaneous values in said storage means and for generating DC content signals for each phase by averaging the stored instantaneous values at said first and second points in each phase.   
     
     
       13. An apparatus as recited in claim 12, wherein said measuring means includes: a multiplexer;   a sample and hold circuit; and   wherein said control means includes means for operating said multiplexer to successively apply each phase of the AC waveform to the sample and hold circuit.   
     
     
       14. An apparatus for generating a signal representative of th DC content of an AC waveform which is subject to frequency variations, said apparatus comprising: a sample and hold circuit;   an analog to digital converter;   a programmable counter being connected to begin counting at a predetermined initial count; and   a digital processor for operating the sample and hold circuit to hold the instantaneous value of the AC waveform at a first point in time, for operating the analog to digital converter to convert the instantaneous value of the AC waveform into a digital signal, for storing the digital signal, for operating the sample and hold circuit to hold the instantaneous value of the AC waveform at a second point in time spaced from said first point by a time interval corresponding to a final count in said programmable counter, said time interval being substantially equal to an exact odd number of half cycles of said AC waveform, wherein said instantaneous values of said AC waveform at said first and second points are equally and oppositely displaced from the mid-value of said AC waveform, for operating said analog to digital converter to convert the second instantaneous value into a second digital signal and for averaging said first and second digital signals to generate a signal representative of the DC content of said AC waveform.   
     
     
       15. An apparatus as recited in claim 14, further including: a multiplexer;   wherein said AC waveform is a multiphase waveform and said digital processor operates said multiplexer to successively apply the instantaneous value of each phase of the multiphase AC waveform at respective first points in a cycle of each phase to said sample and hold circuit for conversion by the analog to digital converter and for storage by the digital processor, operates the multiplexer to successively apply the instantaneous value of each phase of the multiphase AC waveform at a second point spaced from the respective first point by said time interval to the sample and hold circuit for conversion by the analog to digital converter and for storage by the digital processor, wherein said instantaneous values of each phase of said AC waveform of said first and second points are equally and oppositely displaced from the midvalue of the associated phase of said AC waveform, and generates signals representative of the DC content of said AC waveform by averaging said first and second instantaneous values of each phase.   
     
     
       16. An apparatus as recited in claim 15, further comprising: means for biasing said AC waveform by a DC voltage prior to measuring said instantaneous values, said bias voltage being of sufficient magnitude to ensure that the first and second instantaneous values will be of the same polarity; and   wherein said digital processor averages the two biased instantaneous values by adding them together and subtracting a signal, equal to twice the bias voltage, from the sum.